Resources

Children’s School Bus Exposure and
Mitigation Studies

This page updated November 3, 2016

ARB has funded 3 studies
to examine children’s exposures in school buses and possible mitigation
approaches. The three studies are discussed and linked
below. A list of published literature on the topic is also
provided below in the section titled “Other Publications.”

The first Children's School Bus Exposure Study was conducted to
characterize the range of children's exposures to diesel
vehicle-related pollutants and other vehicle pollutants during their
commutes to school by school buses. It was the most comprehensive
school bus exposure study ever conducted. Researchers at the University
of California's Riverside and Los Angeles campuses, measured pollutant
concentrations inside five conventional diesel school buses while
driving actual school bus routes in Los Angeles. For comparison, a
diesel bus equipped with a particulate trap and a bus powered by
natural gas were also included.

Buses were outfitted with dual sets of real-time instruments, which
allowed front versus back and inside versus outside comparisons. The
researchers measured multiple diesel vehicle-related pollutants,
including black carbon and particle-bound PAHs, as well as many other
exhaust pollutants. A tracer gas was used to determine the bus's own
contributions to on-board concentrations. The study measured exposures
inside the buses and did not include tail-pipe emissions tests.

Major Findings:

Measurements indicated that for some buses, significantly higher exposures of
vehicle-related pollutants occurred during the bus commutes than
roadway pollutant concentrations alone would indicate. The high commute
concentrations were a function of several influences:

the high concentrations of pollutants already present on roadways, especially if
traffic was heavy;

the
direct influence of other vehicles being followed; and

the contribution of the bus’s own emissions. The extent of a bus’s own
contribution to these high concentrations appeared to be highest when
windows were closed for the older diesel buses, but bus-to-bus
variability was high.

Recommendations from this report include:

Reducing
school bus-related exposures by assigning the newest and cleanest buses
to the longest routes.

One
method tested for determining the overall tightness of a bus was
effective and should be used to check tightness as buses age.

Self-pollution of buses could be significantly reduced by placing the exhaust outlet above the bus.

Exhaust
from a leader bus can be more significant than self-pollution and thus,
this research further strengthened the recommendation that school buses
should not caravan close together or follow closely behind other
diesel-powered vehicles.

In 2012 ARB sponsored a study entitled "Reducing Air Pollution Exposure in
Passenger Vehicles and School Buses" to explore the application of high
efficiency filtration to reduce in-cabin exposure to particulate
pollution, including PM2.5 and ultrafine particles (UFP).
Automobiles and school buses were equipped with high efficiency filters
rated MERV (Minimum Efficiency Reporting Value) 16 or higher, or HEPA
(High-Efficiency Particle Arrestance) filters.

The results of the study showed that high efficiency filtration is an
effective mitigation strategy for reducing in-cabin exposure to
particles in both cars and buses. Filtration reduced ultrafine
particles in school buses by 88%, black carbon by 84%, and PM2.5 by
55%, and filters were even more effective in
cars.

Other ARB School Bus Programs

School Bus Idling ATCM Program - The California Air Resources
Board has approved an airborne toxic control measure (ATCM) that limits
school bus idling and idling at or near schools to only when necessary
for safety or operational concerns.

Zhu,
Y., Lee, E., 2015. Application of a High Efficiency Filtration
System for Air Pollution Exposure Reduction inside School Buses, Final
report to South Coast Air Quality Management District, Grant no.
A-0090914-0.

Fitz, D.R., Winer A., 2006.
Evaluation of mechanisms of exhaust intrusion into school buses and
feasible mitigation measures. Final report to the California Air
Resources Board, contract no.
03-343.

For
more information about the Children's School Bus Exposure Study, you
may contact Kathleen Kozawa at (916) 323-2999